The present invention is based on Japanese Patent Application No. 2000-217187, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a hydrogen storage alloy having a Laves phase containing vanadium.
2. Description of Related Art
Recently, hydrogen energy is focused as a new energy, and development of the hydrogen storage alloy reversibly absorbing and discharging hydrogen, is promoted elaborately in various fields, such as the hydrogen storage, heat pumps, actuators, and electrodes for secondary batteries. Among the hydrogen storage alloys, a Tixe2x80x94Zrxe2x80x94Mnxe2x80x94V based hydrogen storage alloy has a large rechargeable hydrogen capacity, and the excellent alloy performance.
Conventionally, in order to obtain the alloy, a method of utilizing metals of elemental substances as the raw materials is applied, adjusting the component thereof at the time of melting, and providing the raw material melted mainly by the Ar arc melting method or the high frequency induction melting method into a dye. Moreover, after forming the alloy, a high temperature heat treatment is executed for a long time in order to improve the alloy performance.
In the above-mentioned Tixe2x80x94Zrxe2x80x94Mnxe2x80x94V based hydrogen storage alloy, the oxygen content of the raw materials, in particular, of the vanadium drastically influences the hydrogen absorbing and discharging performance. In the case the content is large, the hydrogen absorbing amount of the alloy is deteriorated. However, since a vanadium has ordinarily a relatively large oxygen content of 10,000 ppm or more, a desired performance cannot be obtained in the case that the vanadium is directly used as the raw material for the hydrogen storage alloy. Therefore an oxygen reduction treatment is required. As a result, the alloy production cost is increased and a problem is caused that realization of a system containing the alloy is disturbed.
Moreover, the heat treatment to be performed after forming the alloy also increases the production cost and the production time, Furthermore, it may lead to oxidization of the alloy so as to deteriorate the performance, and thus a problem of difficulty in handling is also involved.
In view of the circumstances, an object of the invention is to provide a Tixe2x80x94Zrxe2x80x94Mnxe2x80x94Vxe2x80x94Fe hydrogen storage alloy with the excellent hydrogen absorbing and discharging performance by optimizing the components. Another object thereof is to provide a production method for a hydrogen storage alloy capable of producing the alloy efficiently at a low cost.
In order to solve the problems, a first aspect of the invention is a hydrogen storage alloy having Laves phase represented by the general formula: Ti1xe2x88x92xZrxMnwxe2x88x92yxe2x88x92zVyFez, wherein 0xe2x89xa6xxe2x89xa60.5, 0 less than yxe2x89xa60.6, 0 less than zxe2x89xa60.2, and 1.8xe2x89xa6wxe2x89xa62.2.
A second aspect of the invention is the hydrogen storage alloy having a Laves phase according to the first aspect, wherein the content of the oxygen is 5,000 ppm or less.
A third aspect of the invention is a production method for a hydrogen storage alloy having a Laves phase, wherein the hydrogen storage alloy according to the first or second aspect is formed by using a ferrovanadium (alloy of a vanadium and an iron) as one of the raw materials.
A fourth aspect of the invention is the production method for a hydrogen storage alloy having a Laves phase according to the third aspect, wherein the oxygen content of the ferrovanadium is 4,000 ppm or less.
A fifth aspect of the invention is the production method for a hydrogen storage alloy having a Laves phase according to the third or fourth aspect, wherein the melted raw materials are rapidly quenched and solidified.
Hereinafter, the atomic ratio defined in the invention, or the like, will be explained.
Atomic Ratio of the Alloy
Ti: Atomic Ratio 0.5 to 1.0
Since the titanium is an element capable of increasing the hydrogen absorbing amount, it is added as an essential component. However, in order to certainly obtain the above-mentioned effect, the atomic ratio should be 0.5 or more. In contrast, in the case it is added by more than a 1.0 amount, the hydrogen dissociation pressure is lowered. Therefore, the atomic ratio is set in the range of 0.5 to 1.0.
Zr: Atomic Ratio 0.5 or Less
Since the zirconium is an element capable of adjusting the hydrogen equilibrium dissociation pressure, it is optionally added. However, in the case it is added by a more than 0.5 atomic ratio, the hydrogen equilibrium dissociation pressure is lowered. Therefore, the upper limit of the atomic ratio is set at 0.5.
Mn: Atomic Ratio 1.0 to Less than 2.2
Since the manganese is an element capable of lowering the hydrogenation reaction temperature, it is added as an essential component. However, in order to certainly obtain the above-mentioned effect, the atomic ratio should be 1.0 or more. In contrast, in the case it is added by a 2.2 or more amount, the hysteresis is enlarged. Therefore, the atomic ratio is set in the range of 1.0 to less than 2.2.
V: Atomic Ratio 0.6 or Less
Since the vanadium is an element capable of increasing the hydrogen absorbing amount, it is added as an essential component. However, in the case it is added by a more than 0.6 atomic ratio, the reaction rate is lowered. Therefore, the upper limit of the atomic ratio is set at 0.6.
Fe: Atomic Ratio 0.2 or Less
Since the iron is an element contained at the time of using a ferrovanadium, it is added as an essential component. However, in the case it is added by a more than 0.2 atomic ratio, the hydrogen equilibrium dissociation pressure is raised. Therefore, the upper limit of the atomic ratio is set at 0.2.
Impurity Oxygen: 5,000 ppm or Less
The impurity oxygen contained in a hydrogen storage alloy influences the hydrogen absorbing and discharging ability. In the case the amount thereof is large, the absorbing and discharging ability is deteriorated. Therefore, the contained oxygen amount is preferably as little as possible. In consideration of the industrial applicability, the content thereof is preferably 5,000 ppm or less, and further preferably 1,000 ppm or less.
Laves Phase
Since a hydrogen storage alloy according to the invention has a Laves phase structure, it provides a high hydrogen absorbing effect owing to the Laves structure.
Use of Ferrovanadium
Since the alloy of the invention provides a good hydrogen absorbing and discharging performance owing to an appropriate component adjustment (including the iron), the ferrovanadium can be used as the raw material. Since the ferrovanadium is produced at a low cost compared with the case of a vanadium single metal, a desired hydrogen storage alloy can be produced efficiently at a low cost. As the ferrovanadium, for example, those containing a vanadium by 80 to 85% mass ratio, and an iron as the substantially remainder, can be presented. Furthermore, it is desirable that the ferrovanadium has the oxygen included as the impurity, limited to 4,000 ppm or less. More preferably, the ferrovanadium may contain the oxygen not more than 3,000 ppm. According to the limitation of the oxygen content, the oxygen content of a hydrogen storage alloy prepared with the ferrovanadium as the raw material can be sufficiently lowered so that the averse effect to the hydrogen absorbing and discharging performance can be eliminated.
Rapid Solidification
Furthermore, in the production of a hydrogen storage alloy according to the invention, the raw materials with the components adjusted, are melted, and rapidly quenched and solidified for preparation.
By preparing the hydrogen storage alloy by quenching and solidifying, by for example, roll quenching, the plateau properties and the hysteresis properties can be improved dramatically so that the storage and transportation efficiency of the hydrogen can be improved. In the conventional production method, the cooling operation at the time of forming an alloy is carried out by natural cooling or water cooling. In contrast, in the invention, the above-mentioned effect can be formed by quenching and solidifying at a cooling rate higher than that of the conventional method. As to the cooling rate, a cooling rate of 100xc2x0 C./second or more can be presented, and furthermore, a cooling rate of 103xc2x0 C./second or more can be presented as a preferable embodiment.
Particularly in the case of adding a zirconium, segregation of the zirconium in the alloy can be prevented so that a problem of increase of the plateau slope due to the zirconium segregation can be solved.
The above-mentioned method for rapid solidification is not particularly limited, and various methods capable of obtaining the cooling rate can be adopted. For example, the gas atomize method, the centrifugal method, the rotation submerged jetting method, the roll quenching method, or the like, can be adopted.
In the invention, a hydrogen storage alloy is obtained preferably by measuring each of the Ti, Zr, Mn, Fexe2x80x94V component materials so as to have the atomic ratio defined in the invention, melting the same by an ordinary method, and rapid solidification by the roll quenching method, or the like, as mentioned above.
Although a homogenization process of heating the alloy at a high temperature for homogenizing the components is applied in the conventional method after preparing a hydrogen storage alloy, in the invention, a sufficient homogenization effect is obtained by the above-mentioned rapid solidification so that a final alloy can be provided without the need of the homogenization process, which is executed in the conventional method. The final alloy denotes an alloy in the state to be used for the targeted application without the need of a heat treatment, or the like.
The obtained hydrogen storage alloy is pulverized as needed by a mechanical method, or the like so as to provide a powdery hydrogen storage alloy. The pulverization method is not particularly limited, and an optional method such as a known method can be adopted.
The hydrogen storage alloy in the powdery form can be used for a desired application as it is or after shaping. The application of the hydrogen storage alloy obtained by the invention is not particularly limited, and it can be used for various applications utilizing the hydrogen absorbing and discharging phenomenon. For example, it can be used in a heat pump of a heat transportation system or a freezing system, a hydrogen storage system, or the like. By use of the hydrogen storage alloy obtained by the invention in these systems, the system efficiency can be improved dramatically.